Nanostructured (NS) materials formed by severe plastic deformation techniques such as rolling, drawing, and extrusion are much stronger than their coarse-grained (CG) counterparts. However, applications for these types of nanostructured materials are limited because these techniques, which reduce the grain size and increase the strength, also reduce the ductility. As a result, these types of nanostructured materials possess very low (nearly zero) uniform tensile elongation, which results in necking immediately after yielding in tension. The onset of localized deformation in tension is governed by the equation
                                          (                                          ∂                σ                                            ∂                ɛ                                      )                    ɛ                ≤        σ                            (        1        )            where σ is the true stress, and ε is the true strain. The loss of strain hardening during severe plastic deformation results from the reduction of the dislocation storage capacity in the tiny grains of the materials. The loss of dislocation storage capacity in these high strength materials makes them prone to plastic instability (early necking), which leads to lower uniform elongation.
Recent efforts have been made in improving the ductility of nanostructured materials by increasing the dislocation storage capacity of the grains to regain the strain hardening that is lost due to small grain sizes. The strategies employed in regaining the strain hardening generally involve tailoring the microstructures of the materials and changing the tensile conditions. Y. Wang et al. in “High Tensile Ductility in a Nanostructured Metal,” Nature, vol. 419, October 2002, pp. 912-916, for example, describe a method for improving strain hardening in copper by rolling copper at liquid nitrogen temperature to suppress dynamic recovery and allow the density of accumulated dislocations to reach a higher steady state level than what can be achieved at room temperature. Afterward, the material is annealed at a temperature of 180 degrees Celsius. The result is a material having a bimodal grain size distribution of micrometer size grains embedded in a matrix of nanocrystalline and ultrafine grains. The matrix grains impart high strength while the inhomogeneous microstructure induces strain hardening in the material. This method has also been described in U.S. Published Patent Application Number 2004/0060620 to Ma et al. entitled “High Performance Nanostructured Materials and Methods of Making the Same”.
Valiev et al. in “Paradox of Strength and Ductility in Metals Processed by Severe Plastic Deformation,” Journal of Materials Research, vol. 17, no. 1, January 2002, pp. 5-8, describe that treatment of copper by cold rolling to a thickness reduction of 60 percent significantly increased the strength but dramatically decreased the elongation to failure (which is a quantitative measure of ductility). Valiev et al. report that treatment of copper to two passes through an equal channel angular pressing (ECAP) die also increased the strength of the copper but decreased the ductility. However, continued deformation of the copper for a total of sixteen passes through the ECAP die increased both the strength and the ductility, and the increase in ductility was even greater than the increase in strength. Similar results were observed by subjecting titanium to high-pressure torsion. Transmission electron micrographs of the resulting ultrafine-grained materials show that the mean grain size for these materials is about 100 nm for copper and for titanium.
Youssef et al. in “Ultratough Nanocrystalline Copper With a Narrow Grain Size Distribution,” Applied Physics Letters, vol. 85, no. 6, 9 Aug. 2004, pp. 929-931, describe a method of preparing high strength copper with good ductility. The method involves milling copper powder at liquid nitrogen temperature, then flattening the milled powder, and then welding the powder to form thin flakes. Continued milling at room temperature and at cryogenic temperatures induced in situ consolidation of the flakes into fully dense nanocrystalline copper spheres having high yield strength (770 MPa) and good ductility.
Wang et al. in “Tough Nanostructured Metals at Cryogenic Temperatures,” Advanced Materials, vol. 16, no. 4, 17 Feb. 2004, pp. 328-331, describe a method for preparing nanostructured metals by equal channel angular pressing followed by cold rolling. The study found that strength and ductility can be improved at low temperatures and high strain rates. However, the method does not work at room temperature and quasi-static service conditions.
Horita et al. in “Achieving High Strength and High Ductility in Precipitation-Hardened Alloys,” Advanced Materials, vol. 17 (2005) pp. 1599-1602, describe processing an Al-10.8 wt % Ag by first heating the alloy to dissolve second phase particles, then refining the grain size by equal channel angular pressing (ECAP), and then annealing. This process rendered the alloy with both high strength and good ductility.
The strategies used in the above methods attempt to regain the strain hardening of nanostructured materials and improve the ductility by increasing the dislocation storage capacity of the materials. However, improvements in the ductility are always accompanied by a decrease in the strength.
There remains a need for nanostructured materials having improved ductility, but not at the expense of strength.